Cross compiling support with sysrooted toolchains

[Ben Phillips]

Hello List,

I'm a maintainer of a embedded Linux distribution and I am primarily
responsible for each release's cross compiling toolchain, and any target
packages that go into its sysroot. So the typical place to put a sysroot
these days seems to be in the root of the toolchain, so we have something
like this.

<path to toolchain>:
./arm-unknown-linux-gnueabi
./bin    <--- All cross compilers and other utils targeting arm eabi go here
./include
./lib
./libexec
./share
./sysroot <--- All third party libraries go here so we can use them for
further dev

pkg-config is the closest thing we have to seamless configuration, building
and linking with a toolchain in my opinion. I would like to ask your
opinions on a few issues I feel exist in pkg-config.

Before pkg-config version 0.28, it seems that one has to export some
environment variables to ensure that the pc file search path only included
the relevant directories in the sysroot. I may be wrong but something along
these lines:

$ unset PKG_CONFIG_PATH               # empty so no additional paths
searched
$ PKG_CONFIG_LIBDIR=<path to toolchain>/sysroot/usr/lib/pkgconfig     #
could include more paths
$ export  PKG_CONFIG_LIBDIR
$ PKG_CONFIG_SYSROOT_DIR=<path to toolchain>/sysroot
$ export PKG_CONFIG_SYSROOT_DIR

Many maintainers would put that in a wrapper script (inside the bin dir of
the toolchain) called arm-unknown-linux-gnueabi-pkg-config and call
pkg-config at the end of the script. Therefore in an autotools project, you
can configure the project to build correctly with:

$ export PATH=<path to toolchain>/bin:$PATH
$ PKG_CONFIG=arm-unknown-linux-gnueabi-pkg-config ./configure
--host=arm-unknown-linux-gnueabi

The configure script will find <host tuple>-gcc (in my case
arm-unknown-linux-gnueabi-gcc) if it was requested in the
configure.acfile...through the AC_PROG_CC macro. Does
PKG_PROG_PKG_CONFIG do the same?
If not, shouldn't it? Since pkg-config 0.28, we obtain an
arm-unknown-linux-gnueabi-pkg-config program when we configure and build
pkg-config with:

$ ./configure --host=arm-unknown-linux-gnueabi --prefix=/
$ make
$ make DESTDIR=<path to toolchain> install

Looking at the output of "<path to
toolchain>/bin/arm-unknown-linux-gnueabi-pkg-config --list-all --debug" it
is trying to find pc files in /lib/pkgconfig (and some others). If we
change the way we build it:

$ ./configure --host=arm-unknown-linux-gnueabi --prefix=<path to toolchain>
$ make
$ make install

Now "<path to toolchain>/bin/arm-unknown-linux-gnueabi-pkg-config
--list-all --debug" is trying to find the pc files in <path to
toolchain>/lib/pkgconfig (and some others). So it is still not searching
for them in where I would have installed target packages (<the
sysroot>/usr). So before I make this message a tutorial on how I think
pkg-config works, let me compare it to GCC.

The GCC is configured and built with a build tuple, host tuple and target
tuple. The build is usually something like x86_64-pc-linux-gnu and the host
would also be x86_64-pc-linux-gnu if I was building my CROSS compiler for
ARM. That allows me to run arm-unknown-linux-gnueabi-gcc on my x86_64
machine, the thing that tells GCC to produce objects for arm is the target
(arm-unknown-linux-gnueabi). As you can see from the way I configured
pkg-config, host arm-unknown-linux-gnueabi is wrong, but the build
continues successfully if I don't have an arm-unknown-linux-gnueabi c
compiler in the path. In my opinion, pkg-config should run on host and be
used for target.

GCC would also have been configured with --with-sysroot=<path to
toolchain>/sysroot, and GCC would then treat that directory as the root
filesystem for the target, i.e. it is impossible that the c library would
be found in /lib (gcc will only look in <sysroot>/lib and other standard
paths).  The toolchain is also relocatable in this case (just the way GCC
works), which means I can build a toolchain in /home/ben/toolchain and a
guy called Tim can copy it to /home/tim/toolchain and it will still
resolve...am I wrong that arm-unknown-linux-gnueabi-pkg-config would still
be looking in the prefix /home/ben/toolchain?

So in summary, my ideal situation is:
$ ./configure --prefix=/ --target=arm-unknown-linux-gnueabi
--with-relative-sysroot=/sysroot    # sysroot is given relative to the
install prefix
$ make
$ make DESTDIR=/home/ben/toolchain install
$ /home/ben/toolchain/bin/arm-unknown-linux-gnueabi-pkg-config --list-all
someCrazyArmLibrary.pc
...
<Do not show me anything in /usr/lib/pkgconfig...even on Tim's machine>

Thoughts?

Regards,
Ben Phillips
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